Run the protein over a sizing column. It will run at the apparent MW of the protein if it’s a monomer, or some higher MW if it forms multimers. That would be the simplest thing to do. Native gels are possible but tricky to do and interpret. Or you might try mild cross-linking with glutaraldehyde followed by SDS-PAGE.

If you have purified what you believe to be active protein, you can do an N-terminal sequence determination. If it is a monomer or a homodimer-trimer-etc. there will only be one N-terminal sequence. Then your MW estimate (from the sizing column) will tell you if it’s a monomer, dimer, trimer, or whatever. If there is more than one N-terminal sequence present, then it’s a heterodimer or multimer of some sort. It is possible to do a mass spec, too.